fix: improve otel fanout docs discoverability [closes DOC-796]

src/langsmith/trace-with-opentelemetry.mdx

@@ -1,8 +1,10 @@
 ---
 title: Trace with OpenTelemetry
+description: Configure OpenTelemetry tracing in LangSmith, including LANGSMITH_OTEL_ENABLED and OTEL fanout with the OpenTelemetry Collector.
 sidebarTitle: OpenTelemetry
 ---
 
+
 LangSmith supports OpenTelemetry-based tracing, allowing you to send traces from any OpenTelemetry-compatible application. This guide covers both automatic instrumentation for LangChain applications and manual instrumentation for other frameworks.
 
 Learn how to trace your LLM applications using OpenTelemetry with LangSmith.
@@ -591,6 +593,10 @@ Here is an [example](https://smith.langchain.com/public/9574f70a-b893-49fe-8c62-
 
 ### Use OpenTelemetry collector for fan-out
 
+Use `LANGSMITH_OTEL_ENABLED=true` when you need OTEL fanout. Configure your application to emit OTEL spans once, then use an OpenTelemetry Collector to route them to LangSmith and any additional observability backends.
+
+Use this approach when you are tracing applications and want multi-destination routing. If you are operating LangSmith platform infrastructure telemetry (logs, metrics, traces from self-hosted LangSmith services on Kubernetes), use the [Configure your collector for LangSmith telemetry](/langsmith/langsmith-collector) guide instead.
+
 For more advanced scenarios, you can use the OpenTelemetry Collector to fan out your telemetry data to multiple destinations. This is a more scalable approach than configuring multiple exporters in your application code.
 
 1. [Install the OpenTelemetry Collector](https://opentelemetry.io/docs/collector/getting-started/) for your environment.